A. Field of the Invention
The present invention relates to a liquid crystal display (LCD), especially to a TFT (thin film transistors) active matrix LCD having satisfactory visibility in a wide viewing angle range.
B. Description of the Prior Art
Compared with the conventional cathode-ray tube (CRT), the LCD has many advantages, such as, thinner in size and lighter in weight. Nevertheless, the viewing angle characteristic of the LCD is very narrow. To relieve the viewing angle dependence of the LCD, various types of LCD have been developed, for example, IPS (In-Plane Switching) LCD. However, IPS LCD is restricted to the application of desktop computers rather than notebooks since the optical transmission rate of IPS LCD is much lower than that of conventional TN (Twisted Nematic) LCD.
To improve the viewing angle dependence, a conventional method is to provide a LCD wherein there are many orientation directions for the liquid crystal molecules in the liquid crystal layer. For instance, a conventional LCD developed by Fujitsu company is illustrated in FIGS. 6A and 6B. Refer to FIG. 6A, the LCD includes an upper glass substrate 1, a lower glass substrate 2, and a liquid crystal layer 5 interposed therebetween. A pixel electrode 3 is disposed on the upper glass substrate 1 while a partial common electrode 4 is disposed on the lower glass substrate 2. More particularly, there are multiple protrusions 6 on the pixel electrode 3 and the partial common electrode 4. An orientation layer 7 overlays the homeotropic surface which covers the pixel electrode 3 and the protrusions 6. The other orientation layer 7 overlays the homeotropic surface which covers the partial common electrode 4 and the protrusion 6.
The structure as shown in FIG. 6A illustrates the situation when no voltage is applied between the pixel electrode 3 and the partial common electrode 4 or when the voltage applied between the pixel electrode 3 and the partial common electrode 4 is lower than a threshold voltage. Under the circumstance, all liquid crystal molecules have the same orientation direction (the vertical direction in FIG. 6A). On the other hand, when the voltages applied between the pixel electrode 3 and the partial common electrode 4 exceeds the threshold voltage, the behavior of the liquid crystal molecules will vary in a manner as shown in FIG. 6B. Under the circumstance, the orientation directions of the liquid crystal molecules will be affected by the protrusions 6. Consequently, the viewing angle can be widened utilizing the LCD structure as illustrated in FIGS. 6A and 6B.
Another approach is developed by IBM and is illustrated in FIGS. 7A and 7B. Refer to FIG. 7A, the partial structure of the LCD includes an upper glass substrate 1, a lower glass substrate 2, a pixel electrode 3, a partial common electrode 4, a liquid crystal layer 5 and two orientation layers 7.
A groove 9 is formed on the partial common electrode 4, which causes discontinuities of the partial common electrode 4. When no voltage is applied between the pixel electrode 3 and the partial common electrode 4, or when the voltage applied between the pixel electrode 3 and the partial common electrode 4 does not exceed a threshold voltage, the liquid crystal molecules in the liquid crystal layer 5 have the same orientation direction, as illustrated in FIG. 7A. On the other hand, when the voltage applied between the pixel electrode 3 and the partial common electrode 4 exceeds the threshold voltage, the orientation direction of the liquid crystal molecules near the groove 9 will vary in a manner as illustrated in FIG. 7B. It is due to the non-uniform electric fields (as shown by the arrows in FIG. 7B) caused by the existence of the groove 9. Consequently, the viewing angle can also be widened utilizing the LCD structure as illustrated in FIGS. 7A and 7B.
Yet another approach is also developed by IBM and illustrated in FIG. 8. The LCD includes: an upper glass substrate 1, a lower glass substrate 2, a liquid crystal layer 5 interposed therebetween, a pixel electrode 3, a partial common electrode 4, a protrusion 6, and two orientation layers 7. The structure described in FIG. 8 can be considered to be a combination of the structures described in FIGS. 6A, 6B and FIGS. 7A, 7B. Specifically, the orientation directions of the liquid crystal molecules can be further diversified because of the two reasons: (1) the liquid crystal molecules near the protrusion 6 tend to align perpendicular to the homeotropic surface which covers the partial common electrode 4 and the protrusions 6; (2) the fringing field effect (as shown by the arrows in FIG. 8) appears at the edges of the pixel electrode 3. Consequently, the viewing angle can also be widened utilizing the LCD structure as illustrated in FIG. 8.
In summary, for the prior art as illustrated in FIGS. 6A, 6B and FIG. 8, the dielectric materials constituting the protrusions 6 must satisfy some requirements such as resistivity, dielectric constant, and shape. As for the prior art illustrated in FIGS. 7A, 7B, the fabrication processes is somewhat complicated because the groove 9 need to be formed on the partial common electrode 4.